1. Field of the Invention
The present invention relates to a method and means for measuring preloads in assembled mechanisms and, more particularly, to a simple and accurate technique for measuring the amount of compressional load in an assembly of clamped elements without unloading or disturbing the load.
2. Description of the Prior Art
In a wide variety of mechanical mechanisms, it is necessary to assemble a plurality of elements so as to place such elements under a known compressional load, i.e., a preload. For example, in the case of a rolling element bearing assembly for use in a precision machine, it is common to place the bearings under a given preload. This is necessary for a variety of reasons. That is, if there is no load on the bearing, the bearing races are free to move relative to each other and this permits lateral movement of the rolling elements axis. Since the rolling element is typically functioning to guide a moving part, lateral movement of the rotating element axis introduces inaccuracies in its guiding function. Furthermore, the absence of a preload in a bearing assembly can cause undue wear and premature failure of the bearing. In addition, if the machine in which the bearing is assembled vibrates, an impact load is added to the bearing which shortens the useful life thereof.
A preload effectively solves the above problems by eliminating "play" from the rotating member, reducing wear factors on the bearing be required to drive a driven element. With the correct preload, two bearings are placed in opposition to each other within the assembled mechanism to maintain regidity of the supported member.
In the past, it has been difficult, if not impossible, from a practical standpoint, to measure the preload within an assembled mechanism, such as a rolling element bearing assembly, and even more difficult to measure the preload without disturbing the assembly. One prior solution is the use of matched bearing pairs. This involves manufacturing a pair of bearings with dimensions that result in a predetermined deflection in each bearing as they are clamped together. This deflection, along with the spring rate of the bearings, determines the preload within the assembly.
This method is convenient and is widely used. However, it is inaccurate at low preloads, requires extremely precise spacers if the bearings are to be located apart from each other, does not allow changing the preload once the parts are manufactured, and the resulting preload cannot easily be measured.
Another prior art solution involves measuring the deflections of the bearings as the parts are assembled. How far a threaded retainer is tightened or how many shims are installed is determined by the deflection measurement. This method is also widely used. However, it too is inaccurate at low preloads, requires extremely precise deflection measurements, and the resulting preload cannot easily be measured after the mechanism is assembled.
Another prior art solution involves the use of a low rate spring in the preload path. Changes in preload results in large spring deflections which can easily be measured with a calipers or even a ruler. Unfortunately, external forces need only be as high as the bearing preload to cause compression of the spring and large shifts in position of the rotating member.